a review on anaerobic membrane bioreactors: applications

Desalination 314 (2013) 169–188

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Desalination

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A review on anaerobic membrane bioreactors: Applications, membrane fouling andfuture perspectives

Hongjun Lin, Wei Peng, Meijia Zhang, Jianrong Chen ⁎, Huachang Hong ⁎⁎, Ye ZhangCollege of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua, 321004, PR China

H I G H L I G H T S

► Recent progress in AnMBRs treating various wastewaters is summarized.► Advances in membrane fouling control strategies in AnMBRs are addressed.► Research directions regarding AnMBR technology are identified.► AnMBR is a promising technology for wastewater treatment and reuse.

⁎ Corresponding author. Tel.: +86 579 82282273.⁎⁎ Corresponding author. Tel.: +86 579 82283589.

E-mail addresses: [email protected], [email protected] (J. Chen

0011-9164/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.desal.2013.01.019

a b s t r a c t
a r t i c l e i n f o
Article history:Received 8 October 2012Received in revised form 21 January 2013Accepted 22 January 2013Available online 21 February 2013

Keywords:Anaerobic membrane bioreactorMembrane foulingIndustrial wastewaterMunicipal wastewaterMembrane cleaning

In the last years, anaerobic membrane bioreactor (AnMBR) technology is being considered as a very appealingalternative for wastewater treatment due to the significant advantages over conventional anaerobic treatmentand aerobic membrane bioreactor (MBR) technology. Many articles have touted the diverse potential applica-tions of AnMBR in various stream treatment, and membrane fouling issues. In current review, the fundamentalsof AnMBR (including advantages and configurations, membrane materials and modules, and history develop-ment), application development in various stream treatment, andmembrane fouling researches are summarizedand critically assessed. The characteristics of AnMBR and aerobic MBR for wastewater treatment are also com-pared. AnMBR technology appears to be suitable for treatment of various streams, especially for food industrialwastewater and municipal wastewater. AnMBR treatment usually encounters more serious membrane foulingproblem. This, however, can be remedied through various conventional and novel membrane fouling controlor cleaningmeasures. Based on the review, future research perspectives relating to its application andmembranefouling research are proposed.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1702. Fundamentals of AnMBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

2.1. Advantages and disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1702.2. Membrane materials and modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1712.3. History and commercial development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

3. Applications in various wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1723.1. Treatment of various wastewaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

3.1.1. Synthetic wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1723.1.2. Industrial wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1723.1.3. Municipal wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1753.1.4. Other stream treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

3.2. Applications in biogas production and energy recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1773.3. Operational conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1773.4. Applicability of AnMBRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

4. Membrane fouling issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1794.1. Membrane fouling classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

), [email protected] (H. Hong).

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170 H. Lin et al. / Desalination 314 (2013) 169–188

4.2. Membrane fouling mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1804.3. Parameters affecting membrane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1814.4. Membrane fouling control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

4.4.1. Pretreatment of feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1814.4.2. Optimization of operational conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1814.4.3. Modifying broth properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1814.4.4. Membrane optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824.4.5. Membrane cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

5. Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

1. Introduction

Anaerobic digestion is one of themost important processes used forvarious industrial wastewaters as well as sewage treatments because itcombines pollution reduction and energy production. Moreover, com-pared to the aerobic counterparts, the costs of aeration and sludge han-dling in anaerobic treatment are dramatically lower as no oxygen isneeded and sludge yield is lower. Notwithstanding these advantages,the widespread application of conventional anaerobic biological sys-tems has been limited. This is mostly due to the biomass retentiondilemma. Biomass retention is one of themost important aspects of an-aerobic technology providing sufficient solid retention time (SRT) forthe methanogens. On one side, the net biomass production is low, upto ten times less than that of aerobic treatment. On the other side, therelatively poor settling properties of the biomass in conventional anaer-obic biological treatment systemswould result in the loss of biomass toeffluent. This situation corresponds to the poor biomass retention in theconventional anaerobic biological system. While biofilm or granule for-mation offers the strategy for biomass retention in modern high-rateanaerobic reactors (HRARs), it usually requires a long start-up period,and is a complex process that involves physico-chemical as well as bio-logical interactions, and has proven to bemuchmore problematic underconditions of high or low temperature, or for the treatments of lowstrength and/or salinity wastewater [1,2].

Over the past 15 years, the use of membranes in aerobic biologicalwaste treatment processes has been well established. A complete reten-tion of allmicroorganisms in the bioreactor can be achieved inmembranebioreactors (MBRs) by the use of microfiltration (MF) or ultrafiltration(UF) modules. The MBR technology also offers advantages in terms ofreduced footprint, capacity of handlingwidefluctuations in influent qual-ity and improved effluent quality. Since membranes work well withaerobic processes, it should be possible to extend to anaerobic processes.This is of particular interest for anaerobic processes that depend on theretention of a large population of slow growing microorganisms.

Due to its unique advantages, anaerobic membrane bioreactor(AnMBR),which combines anaerobic process andmembrane technology,

Table 1Comparison of conventional aerobic treatment, anaerobic treatment, aerobic MBR and AnM

Feature Conventional aerobic treatment Conventio

Organic removal efficiency High HighEffluent quality High ModerateOrganic loading rate Moderate HighSludge production High LowFootprint High High to mBiomass retention Low to moderate LowNutrient requirement High LowAlkalinity requirement Low High for cEnergy requirement High LowTemperature sensitivity Low Low to moStart up time 2–4 weeks 2–4 montBioenergy recovery No YesMode of treatment Total Essentially

is attracting remarkable interest in both research community andindustrial sectors. A careful literature review shows that more than 250peer-reviewed English papers regarding AnMBR technology have beenpublished, and more than 100 out of them have been published just inthe last 6 years. Although much research has been conducted on thistopic, studies have generally been limited to single treatment system,and there are still some challenging issues regarding AnMBR systems,particularlymembrane fouling problem. Therefore, it is necessary to sum-marize and compare the results obtained in the literature in order to pro-vide an overview of the findings. Several review papers (most wererecent) were available in the literature, which focused on applicationson various wastewater treatment [3] or only on industrial wastewatertreatment [4], parameters governing permeate flux [5], effect of opera-tional conditions [6], and anaerobic bioprocess [7] in AnMBR systems, re-spectively. While these reviews extended our understanding of AnMBR,they didn't comprehensively address application concerns and mem-brane fouling issues, nor did they cover the updated studies simulta-neously. With the rapid development of AnMBR technology, a detailedand comprehensive analysis of past academic research progress couldbe valuable.

The objective of this review was then to conduct a comprehensiveliterature survey to the recent (mainly from 2006 onwards) applicationprogress and membrane fouling issues regarding AnMBR technology.Accordingly, fundamental aspects of AnMBR would be introduced anddiscussed. The developments in applications, researches on membranefouling mechanisms, factors and control measures would also bereviewed and discussed. Finally, the main conclusions and the futureperspectives were presented.

2. Fundamentals of AnMBR

2.1. Advantages and disadvantages

An AnMBR can be simply defined as a biological treatment processoperatedwithout oxygen andusing amembrane to provide solid–liquidseparation. The advantages offered by this process over conventional

BR.

nal anaerobic treatment Aerobic MBR AnMBR

High Highto poor Excellent High

High to moderate HighHigh to moderate Low

oderate Low LowTotal TotalHigh Low

ertain industrial stream Low High to moderateHigh Low

derate Low Low to moderatehs b1 week b2 weeks

No Yespretreatment Total Total or pretreatment

Table 2Main membrane materials and modules used in AnMBR studies.

Membranematerial

Moduleconfiguration

Nominal poresize/μm

Manufacturer Reference

PVDF Hollow fiber 0.04 GE, USA [37]PVDF Hollow fiber 100 kDa Koch, USA [38]PVDF Flat sheet 70 kDa,

140 kDaSINAP, China [9,30,39]

PVDF Tubular 0.03 Norit X-Flow, Inc.Netherlands

[40]

PVDF Tubular 0.1 PCI Membrane Systems,Inc. USA

[41]

PES Flat sheet 20–70 kDa SINAP, China [10]PES Tubular 20 kDa Weir Envig, Paarl,

South Africa[42]

PE Flat sheet 0.4 Kubota Corporation,Japan

[43]

PE Hollow fiber 0.4 Mitsubishi Rayon, Japan [44]PP Hollow fiber 0.45 Sumitomo Electric Fine

Polymer Inc., Japan[25]

PSF Tubular 0.2 Triqua, Netherlands [26,27]Ceramic Tubular 40 kDa Aquatech Memtuf©,

Korea[32]

Ceramic Tubular 0.2 Atech Innovations,Germany

[45]

Metal Tubular 1.0 Fibertech Co., Ltd, Korea [21]

171H. Lin et al. / Desalination 314 (2013) 169–188

anaerobic systems and aerobic MBR are widely recognized [3,8–10].Table 1 presents the comparison of conventional aerobic treatment,anaerobic treatment, aerobic MBR and AnMBR. It is apparent fromTable 1 that AnMBR technology combined the advantages of anaerobictreatment andMBR technology. Among these, the onesmost often citedare: total biomass retention, excellent effluent quality, low sludge pro-duction, a small footprint and net energy production.

AnMBR systems were essentially implemented based on twoconfigurations: external/side-stream configuration and submerged/immersed configuration. Generally, the external configuration pro-vides more direct hydrodynamic control of fouling, and offers theadvantages of easier membrane replacement and high fluxes butat the expense of frequent cleaning and high energy consumption(of the order 10 kWh/m3 product) [11]. Moreover, high cross-flowvelocity has been reported to have a negative impact on biomass ac-tivities in AnMBR systems [12–14]. Compared to external configura-tion, submerged configuration directly places the membrane intothe liquid. A pump or gravity is used to drag the permeate throughthe membrane. Several distinct advantages of submerged configura-tion are their much lower energy consumption and fewer rigorouscleaning procedures, as well as the milder operational conditionsdue to the lower tangential velocities.

2.2. Membrane materials and modules

The membrane materials can be classified into three majorcategories: polymeric, metallic and inorganic (ceramic). Ceramicmembranes can be backwashed effectively providing high resistanceto corrosion, abrasion, and fouling as well as increased concentrationpolarization control [15,16]. Ghyoot and Verstraete [14] found that acommercial ceramic MF membrane reached 200–250 L/m2/h (LMH),which was 10-fold higher than the flux achieved with a polymer UFmembrane, with both membranes producing permeate of similar qual-ity for filtration of anaerobic sludge. In this respect, ceramicmembranesappeared to be most widely used in early studies regarding AnMBR[14,17–19]. Meanwhile, metallic membranes have also been used inthe AnMBR system, showing better hydraulic performance, better foul-ing recovery, and higher strength endurable impact force and toleranceto oxidation and high temperature compared to polymeric membrane[20,21]. However, ceramic or metallic membranes are much more ex-pensive than polymeric membranes. As economics of a system wasgradually becoming a great concern (this is particular true for commer-cial applications), polymeric membranes gained more interests in bothresearch community and commercial applications in recent years. Thepreferred polymeric membrane materials are polyvinylidene difluoride(PVDF) and polyethersulfone (PES), which account for around 75% ofthe total products on the market including 9 out of the 11 most com-mercially important products [22]. Other polymeric materials, suchas polythylene (PE) [23], polypropylene (PP) [24,25] and polysulfone(PSF) [26,27], are also used for some cases of AnMBR applications.

Most membrane modules used in AnMBRs are implemented byusing MF or UF membranes, with the configuration of either hollowfiber, flat sheet (plate or frame) or tubular. Due to their high packingdensity and cost efficiency, hollow fiber membrane modules are mostpopularly used in SMBRs. However, flat sheet membrane modulesalso retained significant interests, especially from research community[9,28–31], for their advantages of good stability, and the ease of cleaningand replacement of defectivemembranes. A tubularmembranemoduleis made up of several tubular membranes arranged as tubes. The mainadvantages include low fouling, relatively easy cleaning, easy handlingof suspended solids and viscous fluids and the ability to replace orplug a damaged membrane, while the disadvantages include highcapital cost, low packing density, high pumping costs, and high deadvolume. Its applications in AnMBR can be found in many literaturestudies [32–36]. Table 2 summarized the main membrane materialsand modules used in AnMBR studies. As can be seen from Table 2,

most membranes used have pore size ranged from 0.03 to 1.0 μm,which obviously lower than the size of the most flocs or microorgan-isms in AnMBR, and therefore can almost completely retain biomass.

Lin et al. [30] reported that membrane costs accounted for 46.4–72.3% fraction of total capital costs of a full scale AnMBR treatingmunic-ipal wastewater under different assumptions, indicating significantcosts of introduction of membrane modules into the anaerobic systemdue to the relative high costs of membrane. Since membranes onlyserve for solid and liquid separation, and an improved effluent qualitymight not always be required, developing low cost filters applied inAnMBRs would be very desirable. The low-cost filters investigatedinclude non-wovens, meshes and filter cloths as summarized by Menget al. [46]. Although some applications were based on aerobic MBRs, ithas been confirmed that these filters can also be applied in AnMBRs[47,48]. These filters generally have large pore size or porosity, andtherefore could obtain a high initial flux even at a very low pressure[49], but should have a shorter lifetime as compared with polymericmembranes due to their lower tensile and tear strength. Moreover, ap-plication of these filters in AnMBRs encountered the severe foulingproblem [48], and this was mainly caused by the inadequate foulingcontrols, and also can be attributed to their rough surface and the toolarge pore size. It has been reported that precoating the filter clothwith powdered activated carbon (PAC) could mitigate membrane foul-ing [50]. It also indicates that the severe fouling of low-costfilters can beresolved by modifying the filters to improve the surface roughness, hy-drophilicity, surface charge and so on [46]. Meanwhile, self-formingdynamic membranes, which employed cheap coarse pore-sized mate-rials such as Dacron mesh [51,52], non-wovens [53], stainless steelmesh [54], etc., as filtration media, have been extensively investigated.The sludge cake layer and gel layer that dynamically formed on thefiltration medium were found effective in enhancing the solid–liquidseparation, and the effluent quality could be kept at a stable level withundetectable suspended solid (SS) concentration [51,52], suggesting apromising material for separation in AnMBRs.

Another promising membrane process would be forward osmosis(FO). FO is an osmotic process that uses a semi-permeable membraneto effectively separate water from dissolved solutes by using high con-centration draw solution. Because FO uses the osmotic pressure differen-tial across the membrane, rather than hydraulic pressure differential asthe driving force for transport of water through the membrane, it pro-vides recognized advantages including operating at low or no hydraulic


pressures, high rejection of a wide range of contaminants, and lowermembrane fouling propensity as compared to conventional pressure-driven membrane processes [55], and has emerged as an alternativemembrane process to the conventional membrane processes in the re-cent years [55]. Holloway et al. [56] used FO process for concentrationof anaerobic digester centrate, and found that high water flux (initialflux=10.5 LMH) and high nutrient rejection (>90% nitrogen and phos-phorus rejection) could be achieved, showing the potential of using FOprocess in AnMBR. However, the high costs of FO membrane or processmust be reduced to improve its economic feasibility. Also, effect of saltaccumulation on biological activity should be addressed.

2.3. History and commercial development

The AnMBR concept appears to be firstly reported by Grethlein [57]who used external cross-flow membrane to treat septic tank effluent,and achieved increased biomass concentration with 85–95% biochemi-cal oxygen demand (BOD) reduction and 72% nitrate removal simulta-neously. With 3 decades development, the advantages of the AnMBRsystems have been well proven in the literature. Recognizing the valueof AnMBR, both the private sectors and the governments have madeconsiderable investments in promoting AnMBR systems. The notable ef-forts were the development of commercially-available AnMBR systemsknown as the “Membrane Anaerobic Reactor System (MARS)” [58] and“Anaerobic Digestion Ultrafiltration (ADUF)” [59] in the 1980s. Thesesystems have been tested and operated in pilot- and full-scale, andmostly used for industrial wastewater treatment. During the same peri-od, Japan government initiated a national project “Aqua-Renaissance'90”which led to the development of a wide variety of AnMBR systems[60–62]. These commercially-available AnMBR systems were mostlyimplemented based on external configuration. By the 2000s, studieson the AnMBR focused on system performance, filtration characteristics,characterization of membrane foulants, and membrane fouling control.

The success of submerged aerobic MBRs in the early 2000s highlyencouraged the exploration of submerged AnMBRs (SAnMBRs) forwastewater treatment. In the last decade, Kubota Corporation devel-oped a SAnMBR named “KSAMBR” process, which has been successfullyapplied in a number of full-scale food and beverage industries [31].Using the similar technology, ADI Systems Inc. developed ADI-AnMBRsystem specific for food wastewater treatment. The largest AnMBRinstallation up to date in the world was completed by ADI, which pro-duced effluent free of suspended solids (SS) and with 99.4% COD re-moval, allowing 100,000 gal/d of wastewater to be easily dischargedinto the municipal system [63]. Later in 2010s, the submerged AnMBRtreatment was significantly studied with attempts made to improveenergy efficiency, extend the application scope and solve technical prob-lems such as membrane fouling.

3. Applications in various wastewater treatment

3.1. Treatment of various wastewaters

A detailed review shows thatmore andmore attention and efforts ofindividuals and research organizations have been dedicated in AnMBRresearch, especially in the last 6 years. This situation may be attributedto two trends of wastewater treatment. On one side, the industrial sec-tors have been facing with stringent requirements on its increasingwater use efficiency and closing industrial process water cycles andthe same trend will continue in the future. Meanwhile, the extremeconditions of wastewater are likely to become more common in theseyears and in the future. On the other side, while the costs for conven-tional technologies are slowly rising with labor costs and inflationarypressures, the costs for all membrane equipment have been fallingsteadily during the last decade. Moreover, biogas recovery associatedwith AnMBR treatment can create benefits which will significantly off-set the operational costs. On a capital and operational cost basis for any

given project, the likelihood of AnMBR becoming a favored option isincreasing with time. In this section, these AnMBR applications willbe reviewed together with their state of the art in the wastewatertreatment.

3.1.1. Synthetic wastewater treatmentIt is a common operation using synthetic wastewater to test new

concepts or study general aspects of membrane fouling [3]. Most recentstudies regardingAnMBRused syntheticwastewater as feed. This is rea-sonable, considering that AnMBR, especially SAnMBR, is kind of a novelsolution for wastewater treatment, and membrane fouling is a majorissue of AnMBR research. Table 3 presents some recent relevant refer-ences regarding AnMBR systems treating synthetic wastewater. Varioussubstrates have been used to make feed, including glucose, starch,molasses, peptone, yeast, and volatile fatty acids. Due to the absenceof refractory compounds, the chemical oxygen demand (COD) removalby AnMBR was generally higher than 95%. The applied organic loadingrate (OLR) varied depend on research purposes. OLR was generallyhighwhen syntheticwastewaterwas used to test the removal efficiencyor processing capacity of an AnMBR. In theory, AnMBR can achievesame high OLR (usually >10 kg COD/m3/d) achieved by HRARs, suchas upflow anaerobic sludge blanket (UASB) reactors, hybrid UASB reac-tors, and expanded granular sludge bed (EGSB) reactors. However,most studies regarding AnMBR applied OLRb10 kg COD/m3/d. Thiscan be attributed to several aspects associated with the operation ofAnMBRs. By far, most of AnMBRs studied used a completely stirred tankreactor (CSTR) configuration due to the ease of use and construction.Such a configurationwas usually operated at a lower biomass concentra-tion compared to HRARs, corresponding to a lower OLR. Moreover, forresearch studies, it may not be necessary to operate AnMBRs with highbiomass concentration and OLR since membrane fouling was of themain research focus and high biomass concentration or OLR wouldhinder the sustainable AnMBRs operation. From these studies, it shouldbe concluded that AnMBR is a promising technology in terms of highorganic degradation.

3.1.2. Industrial wastewater treatmentRapid industrialization has resulted in the generation of a large

quantity of effluents which include the major sources of industrialwastewaters from food processing, pulp and paper, textile, chemical,pharmaceutical, petroleum, tannery, and manufacturing industries.Industrial wastewater is usually characterized by high organic strengthand/or extreme physical–chemical nature (e.g., pH, temperature,salinity), and containing synthetic and natural substances that maybe toxic to and/or inhibit biological treatment processes.

Table 4 summarizes some significant recent examples of AnMBRapplied to treat some kind of industrial wastewaters. The most popularapplication area appears to be food industrial wastewater. A review ofliterature showed that wastewaters from food industry are generallybiodegradable and nontoxic, and have high concentrations of COD andSS [69]. Liao et al. [3] stated that the extensive opportunity for AnMBRis to treat high organic strength and highly particulate wastewater.The characteristics of food industrial wastewater render it much moresuitable for AnMBR treatment. Generally, COD removal efficiencyachieved was higher than 90%, while the applied OLR was in the rangeof 2–15 kg COD/m3/d. As most of the applied AnMBRs used CSTR con-figurations, the achievable OLR would be lower than the HRAR, buthigher than the conventional CSTR digesters. For instance, KubotaCorporation developed a SAnMBR system named “KSAMBR” process,which has been successfully applied in a number of full-scale food andbeverage industries [31]. The process has the volume which can bescaled down to around 1/3 or 1/5 of the conventional digesters providedthat biomass is 3 to 5 times as concentrated, corresponding to 3 to 5times OLR based on volume if the same flow rate applied.

Treatment of pulp and paper industry wastewater by AnMBR hasbeen reported at least for 10 times, and recent studies were mostly

Table 3Summary of AnMBR performance for synthetic wastewater treatment.

Type of wastewater Scalea Configuration Characteristicsof membraneb

Type ofreactorc

Reactorvolume (L)

Operatingcondition

Influentd Effluente Reference

Tapioca starchwastewater

L External Hollow fiber UFmembranePore size: 0.03–0.15 μm

AF+M 1 HRT=10 dTemp=30 °COLR=1.76 kg COD/m3/d

COD=20.15 COD=675–780(>95%)

[64]

Meatextract+peptone

L Submerged Flat-sheet PE membrane,Pore size: 0.4 μm

CSTR+M 3 HRT=6 h SRT=150MLVSS=2.62±0.13 g/LTemp=35±1 °CFlux=10 LMH

COD=0.45±0.02

CODs=18±9(95%)

[8]

Whey+sucrose L Submerged Flat-sheet membrane,Pore size: 0.4 μm

CSTR+M 11 SRT=30–40 dMLVSS=5.5–20.4 g/LOLR=1.5–13 kgCOD/m3/dTemp=35±1 °CFlux=2–5 LMH

– – [65]

Glucose+peptone+yeast extract

L External Tubular MF membraneZirconia pore size: 0.14 μmPP pore size: 0.2 μm

CSTR+M 4.5 HRT=6.5 dTemp=54–56 °COLR=4 kg COD/m3/d

COD=27.0Kj–N=1.288

CODt=– (78.5–84.4%)

[28]

Glucose L External Hollow fiber PEmembranePore size: 0.4 μm

CSTR+M 25 MLSS=3.5 g/L COD=0.8 – [66]

Glucose L External Flat-sheet membrane,Pore size: 0.45 μm

CSTR+M 5 HRT=12 hSRT=30 dFlux=5.3 LMHOLR=1.1 kgCOD/m3/d pH=6.8–7.0MLVSS=5.132 g/LTemp=25–30 °C

COD=0.55 COD=– (99.1%) [67]

Volatile fatty acid L External Tubular ceramic aluminumoxide (Al2O3) membranePore size: 0.2 μm

CSTR+M 2 SRT=120 dMLVSSb21 g/LOLR=10–55 kgCOD/m3/dTemp=55 °CFlux=20–40 LMH

COD=10 – [45]

Maltose+glucose+volatile fatty acid

L Submerged Hollow fiber PP membranePore size: 0.45 μm

CSTR+M 0.6 HRT=14 dMLVSS=19.5 g/LOLR=2.5 kg COD/m3/dTemp=35 °C

COD=25 COD=95.1±8.6(99.6±0.0%)

[25]

Molasses L External Tubular ceramic membrane,Pore size: 0.1 μm

CSTR/CSTR+M

3/6 HRT=16/32 hMLVSS=1.8/10 g/LOLR=14.9/5.6 kgCOD/m3/dTemp=55/55 °CpH=5.5/7.2

COD=10.2/7.5

COD=- (78–81%)

[68]

a L=laboratory/bench scale.b PE=polythylene and PP=polypropylene.c CSTR=completely stirred tank reactor and AF=anaerobic filter.d The concentration unit is g/L if not specified and – indicates value not reported.e The concentration unit is mg/L; removal efficiency is presented in parentheses; CODs=soluble COD, and CODt=total COD.


conducted by Lin and Liao group [9,39,70–72]. Evaporator condensate(EC), one of the important wastewaters produced from pulp andpaper industry, is characterized as high temperature, high organicstrength (due mainly to methanol), low SS (b3 mg/L), plus inhibitivematerials such as total reduced sulfur (TRS) compounds and turpeneoils [62]. Xie et al. [71] used a SAnMBR operated at 37±1 °C to treatkraft EC for 9 months. Under tested OLRs of 1–24 kg COD/m3/d, a CODremoval efficiency of 93–99% was achieved. Wastewater from pulpand paper industry is usually high temperature, therefore, operationat thermophilic temperatures is of great interest because pre-coolingand post-heating used in the mesophilic treatment for subsequentreuse of treated effluent could be avoided. Lin et al. [9] compared twoparallel SAnMBR treating kraft EC which were operated at mesophilic(37 °C) and thermophilic (55 °C), respectively, and found that a CODremoval efficiency of 97–99% with good methane production wasachieved at a feed COD of 10,000 mg/L in both SAnMBRs. The resultsindicated that both the mesophilic and thermophilic SAnMBRs can bepotentially promising technologies for kraft EC treatment in terms ofCOD removal and biogas production. However, thermophilic SAnMBRsfaced challenge of sever membrane fouling because a high temperature

induced more release of SMP and disruption of sludge flocs [9].Thermomechanical pulp (TMP) is produced by refining wood chips attemperatures above 100 °C, and TMP whitewater is warm, normallywith temperatures between 50 °C and 80 °C, with a COD of 1000–5600 mg/L [73]. Gao et al. [39] investigated TMP whitewater treatmentwith a SAnMBR at an average OLR of 2.4 kg COD/m3/d. Without pHshocks, the steady-state COD removal efficiency was found to beabout 90%, yielding an effluent with CODb300 mg/L. The total cost ofAnMBR for treatment of kraft mill effluent was found to be muchlower than that for aerobic treatment [61,62]. The capital and operatingcosts of an aerobic MBR operated at high-temperature (60 °C) for foulcondensate treatment were significantly lower than the operationalcosts of a steam stripping system [74]. AnMBR treating petrochemicaleffluent has been reported twice in last 6 years, one is laboratory-scale[75] and the other is pilot-scale [76]. Fischer–Tropsch Reaction Water(FTRW) is a typical petrochemical wastewater characterized by highstrength and consisting of short chain organic acids other oxygenateswith a low pH. It was convincingly proven that anaerobic granules didnot readily form with FTRW, and the fixed media systems had effluentquality concerns [76]. AnMBR guaranteed completed biomass retention,

Table 4Summary of AnMBR performance for industrial wastewater treatment.

Type of wastewater Scalea Configuration Characteristicsof membraneb

Type ofreactorc

Reactorvolume (L)d

Operatingcondition

Influente Effluentf Reference

Cheese whey L External MF pore size:0.2 μm

CSTR/CSTR+M

5/15 HRT=1 d/4 dSRT=–/29.7–78.6 dMLVSS=–/6.4–10 g/LOLR=–/19.78 kgCOD/m3/dTemp=37±2/37 °CFlux=139.5 LMH

COD=68.6±3.3BOD5=37.71±2.84Kj–N=1.12±0.01TP=0.5±1.8×10−3

TSS=1.35±0.06pH=6.5

COD=– (98.5%)BOD5b100 (99.2%)TSS=– (100%)

[78]

Diluted tofuprocessing waste

L External Hollow fiberMFmembrane

CSTR+M 5 HRT=4 h RNA concentra-tion=150–200 mg/LTemp=60±0.1 °CpH=5.5±0.1Flux=4.32 LMH

COD=26.5±2.2NH4

+–N=0.86±0.12PO4

3−–P=0.58±0.06TSS=23.5±3.5pH=1.0

Carbohydratecontentb2 g/L

[79]

Olive-millwastewater

L External CeramictubularUF 25 kDaMWCO

PABR+M 15 HRT=16.67 hMLSS=1.05–2.41 g/LTemp=35±2 °CFlux=80–450 LMH

COD=350–500NH4

+–N=15–21PO4

3−–P=3–4.5SS=1–1.5pH=6.5–7.8

CODb30 (>95%)TN=9–9.85(15–20%)PO4

3−–Pb1 (81%)pH=6.9–7.3

[35]

Brewerywastewater+surplusyeast

L External Ceramictubular;Pore size:0.2 μm

CSTR+M 4.5 OLR=12 kg COD/m3/dMLVSS=12 g/LFlux=4–20 LMHTemp=30 °C pH=6.9

CODs=21Particulate COD=45–50

COD=190 (99%)TSS=0 (100%)

[34]

High-concentrationfood wastewater

P External Flat-sheet PES20–70 kDaMWCO

CSTR+M 400 HRT=60 h SRT=50 dpH=7.0±0.2OLRb4.5 kg COD/m3/dMLSS=6–8 g/LTemp=37±0.5 °C

COD=2–15SS=0.6–1.0Chromaticitycolor=6000–1000pH=5–6

COD=141–2388(81.3–94.2%)

[10]

Distilley produceswastewater

F Submerged Kubotaflat-sheetmembrane

CSTR+M – Themophilic range COD=101.3TN=3.72TS=6.0% pH=4.11

COD=– (75–92%) [31]

Kraft evaporatorcondensate

L Submerged Flat-sheetPVDFmembrane140 kDaMWCO

UASB+M 10 HRT=5.8 d SRT=230 dMLSS=8.3±1.6 g/LOLR=3.1±0.8 kgCOD/m3/dTemp=55±1 °CFlux=2.4±0.6 LMH

CODt=10 CODs=– (97–99%) [9]

Kraft evaporatorcondensate

L Submerged Flat-sheetPVDFmembrane140 kDaMWCO

UASB+M 10 HRT=1.93 d SRT=230 dMLSS=8.2±1.5 g/LOLR=12.2±1.1 kgCOD/m3/dTemp=35±1 °CFlux=7.2±0.9 LMH

CODt=10 CODs=– (97–99%) [9]

TMP whitewater L Submerged Flat-sheetPVDFmembrane140 kDaMWCO

UASB+M 10 MLSS=5.7±0.8 g/LOLR=2.4±0.4 kgCOD/m3/dTemp=35±1 °CFlux=5.2±0.5 LMH

CODs=2.78–3.35 CODsb300 (90%) [39]

Petrochemicalwastewater

L Submerged Kubota flatpanelmembranePore size:0.45 μm

CSTR+M 23 HRT=31.5 h SRT=175 dMLSS>30 g/LOLR=14.6 kgCOD/m3/dTemp=37 °CFlux=8.5–16 LMH

COD=19.1 pH=7.2 COD=612 (98%) [75]

Textile wastewater L Submerged Hollow fiberMF mem-branePore size:0.40 μm

CSTR+M 3.25 HRT=24 h pH=6.8–7.2Temp=35 °CFlux=1.8–14.4 LMH PACdose=1.7 g/L

COD=730–1100 COD=– (90%)Color=– (94%)Turbidity=8 NTU

[80]

a L=laboratory/bench scale, P=pilot scale, and F=full scale.b PVDF=polyvinylidine fluoride and PES=polyethersulfone.c CSTR=completely stirred tank reactor, UASB=upflow anaerobic sludge blanket, and PABR=periodic anaerobic baffled reactor.d – indicates value not reported.e The concentration unit is mg/L if not specified; CODs=soluble COD, and CODt=total COD.f The concentration unit is mg/L and removal efficiency is presented in parentheses.


andOLRup to 25 kg COD/m3/dwas achievedwith effluent CODnormallyb500 mg/L with no particulates >0.45 μm [75]. Moreover, no notewor-thy deterioration in membrane performance has been observed overthe 320 d operational period when operated at a low membrane flux of1.5–3.5 LMH [75]. Textile treatment by using AnMBR has been reportedonly once [77]. A SAnMBR combined with PAC addition could achieve

the median removal efficiencies of COD and color with 90% and 94%,respectively [77].

The cases of using AnMBR system for treatment of other industrialwastewaters were very limited. More often, the combinations of anaer-obic unit and aerobic MBR were applied. For refractory wastewaters,anaerobic treatment governed by hydrolysis and acidification is usually


proposed to ameliorate the biodegradability of wastewater feed toaerobic MBR [81]. Such combined systems have been tested for treat-ments of textile wastewater [82], pharmaceutical wastewater [83], oilrefinery wastewater [84], and coke plant wastewater [85]. Enhancedremoval of contaminations was essentially evidenced in these studiesas compared to the single unit. Meanwhile, anaerobic treatment with-outmembranes has been applied successfully to treat various industrialwastewaters [86], As long as a wastewater was amenable to anaerobictreatment, in theory, an AnMBR could be used to treat it [3]. In thiscontext, additional attentions should be paid to improve membraneperformance and the economic feasibility of this technology.

3.1.3. Municipal wastewater treatmentHistorically, anaerobic processes have been mainly employed for

industrial or high strength wastewater treatment while less employedfor municipal wastewater treatment [19,87]. This may mainly due to 2issues. The first one is the difficulty in retaining slow-growth anaerobicmicroorganisms with short hydraulic retention time (HRT) associatedwith treatment of low strength wastewater like municipal wastewater.The second one is that anaerobic effluents rarely meet discharge stan-dards for wastewater reuse due to the kinetic limitations of anaerobicmetabolism [32]. The combination of membrane separation technologyand an anaerobic bioreactor may allow for a sustainable municipalwastewater treatment with complete biomass retention, the addedbenefits of lower sludge production, enhanced high quality effluent,net energy production, andwithout the extra costs for aeration associat-ed with the aerobic treatment processes [87–90]. AnMBR technology isbecoming increasingly popular for municipal wastewater treatment inrecent years [48,88,90].

There are many cases in the literature investigating the efficiency ofthe AnMBR technology for the treatment of municipal wastewater.Table 5 exemplifies recent researches on the application of AnMBRtreatingmunicipalwastewater.With respect to the removal of commoncontaminants, AnMBR systems could typically eliminate around >85%COD, and >99% TSS at selected operational conditions regardless ofthe configurations. The removals were much higher than those of theconventional UASB sewage treatment which usually resulted in a BODremoval efficiency of 80%, effluent COD of 100–220 mg/L, and effluenttotal suspended solids (TSS) of 30–70 mg/L [91], and comparable withaerobic MBR treatment. This is probably not surprising, consideringthat the typical pore sized of the membrane used was in the range of0.01–0.45 μm (Table 5), the SS, most colloids and some organic matterscould be readily retained by the membrane and the cake layer formedon the membrane surface. Due to the complete retention of sludge bythe membrane and application of longer SRT (e.g., 217 d [89]), theretained pollutants may be efficiently removed in AnMBRs. COD remov-al will decrease when membrane pore size increases. This is apparentfrom Zhang et al.'s study [52] where a reduced COD removal of 57.3±6.1% was observed due to the utilization of dynamic membrane forseparation.

In contrast to the high COD and TSS removal, the removal of totalnitrogen (TN) or total phosphorus (TP) in the AnMBR systems is usuallynegligible (Table 5). The low removal of TN and TP is expected becauseboth of TN and TP removal processes required anoxic or aerobic zone.This can be beneficial if the effluent is to be used for agriculture or irri-gation purpose. However, in most cases, this means that the down-stream treatment is needed if the effluent is to be reclaimed. CouplingAnMBRs with conventional biological nutrient removal treatment tech-nologies will face challenges due to the low COD:N and COD:P ratiostypical of AnMBR effluents. Partial nitritation/nitrification would be apromising solution for nutrient removal because ammonium couldserve as the electron donor, and no additional carbon source/electrondonor is required in such process [93]. FOmembrane process could pro-vide another perspective to resolve this challenge since FO process canalmost totally reject N and P contaminants. Physical/chemical nutrient

removal processes could be other solutions although they are signifi-cantly more energy intensive than biological treatment.

The occurrence of trace contaminants such as endocrine disruptingchemicals (EDCs) and pharmaceutically active compounds (PhACs) intreated and untreated municipal wastewater has recently become asignificant environmental health concern [94]. It has been reportedthat removal rate of the EDCs and PhACs during anaerobic digestionis low [95,96]. Ifelebuegu [96] reported the EDCs persisted in the anaer-obic digestion process with percentage removal of 21–24% for steroidalestrogens (E1), 18–32% for 17β-estradiol (E2), 10–15% for 17α-ethynylestradiol (EE2) and 44–48% for nonylphenol (NP). It is worthnoting that prolonging HRT and bioaugmentation would improve theremoval efficiency. Under anaerobic conditions and relatively longHRT (30 d), some PhACs (acetylsalicilic acid (ASA), ibuprofen (IBU),fenofibrate (FNF)) can be significantly degraded [97]. Saravanane andSundararaman [98] applied an AnMBR system to treat pharmaceuticalwastewater containing cephalosporin derivative, and achieved enhanceddegradation (attained a removal of 81% at a maximum cephalosporinconcentration of 175 mg/L) through bioaugmentation. The principalmechanism of removal of these trace contaminants during the sludgeprocess has been demonstrated to be biodegradation bymicroorganismsand also sorption onto biomass [96].

With respect to operational conditions, HRT of AnMBR is generallylonger than 8 h (Table 5), comparing favorably with conventional an-aerobic systems [3], while longer than 4–8 h for aerobic MBRs, whichcorresponds to a less OLR (b3 kg COD/m3/d) as compared to aerobicMBRs. The sustainable membrane flux applied in most AnMBR studiesappeared to be lower than 15 LMH. In contrast, this value for aerobicMBR ranged from 25 to 140 LMH and 3.7–85 LMH for external and sub-merged configuration, respectively [69]. The low sustainablemembraneflux would be a bottleneck to the practical engineering application ofAnMBR. Through the formation process of dynamic membrane on theDacron mesh (pore size=61 μm), a high flux of about 65 LMH wasachieved at an anaerobic dynamic membrane bioreactors (AnDMBR)[52]. Given the relative high cost of UF or MF membranes, and theirlow sustainable flux achieved, AnDMBR seems to be a promising solu-tion for municipal wastewater treatment.

Itwas found that the unit capital costs of SAnMBR treatingmunicipalwastewater was about 800 US$/m3/d capacity [30], which comparesfavorably to the literature values for full-scale aerobic MBRs [99]. Thetotal operational cost value was only 1/3 of the aerobic counterpart atthe similar capacity [100]. Moreover, operational costs can be totallyoffset by the benefits from biogas recovery. Cost sensitive analysisshowed that membrane parameters including flux, price and lifetimeplay decisive roles in determining the total life cycle costs of theSAnMBR [30]. SAnMBR can be a promising technology for municipalwastewater treatment, provided that membrane performance is signif-icantly improved.

3.1.4. Other stream treatmentOther streams, which have been used in treatment by AnMBRs, can

be mainly classified into two categories: high-solid-content streamsand leachate. The former includes wastewater treatment plant sludge,the organic fraction of municipal solid waste, animal processing planteffluents, and manures. It is widely accepted that the hydrolysis orsolubilization stage represents the rate-limiting step in the anaerobicdegradation of most solid organic materials [47]. Hydrolysis proceedsslowly even at optimal conditions, and thus long SRT are required. Forthe conventional anaerobic digestion process which does not decoupleSRT from HRT, long SRT means a large reactor volume and lower OLR,and thus reduces its competitiveness.

It can be seen fromTable 6which summarizes AnMBR applications inthe high-solid-content streams, the applied HRT ranged at 1.5–11.8 d,which was rather higher than the values applied in industrial or munic-ipal wastewater treatment. This indicated that for particulate streamtreatment, a relatively long HRT may be necessary to ensure significant

Table 5Summary of AnMBR performance for municipal wastewater treatment.

Type ofwastewater

Scalea Configuration Characteristicsof membraneb

Type ofreactorc

Reactorvolume(L)

Operating condition Influentd Effluente Reference

Municipalwastewater

L Submerged Flat-sheet MFPVDF 140 kDa MWCO

CSTR 60 HRT=10 hMLSS=6.4–9.3 g/LOLR=~1.0 kgCOD/m3/dTemp=30±3 °CFlux=11 LMH

COD=425±47NH4

+–N=32.4±11.6NO3

−–N=1.3±0.4TP=4.3±0.5SS=294±33pH=7.6±0.3

COD=51±10 (88±2%)NH4

+–N=31.1±12.3 (~0%)NO3

−–N=1.1±0.6 (~0%)TP=3.8±0.7 (~0%)SSb0.8 (>99.5%)pH=7.0±0.2

[30]

Municipalwastewater

L Submerged Flat-sheet dynamicmembrane,Dacron mesh

UASB 45 HRT=8 hMLSS=5.9–19.8 g/LOLR=~0.9 kgCOD/m3/dTemp=10–15 °CFlux=65 LMH

COD=302.1±87.9NH4

+–N=37.9±8.6TN=58.8±10.2SS=120±23pH=7.3±0.3

COD=120.8±34.0(57.7±4.6%)SS=0–15pH=7.2–7.6

[52]

Municipalwastewater

L External Tubular UFmembrane40 kDa MWCO

UASB 1 HRT=3 hSRT=100 dTemp=25 °CFluxb7 LMH

CODt=646±103CODs=385±63TSS=140±18MPNFecal coliforms=106/100 ml

CODt=104±12 (87%)CODs=104±12 (73%)BOD=32±5 TSSb1MPNFecal coliforms=0 (100%)

[32]

DiluteMunicipalWastewater

L External PVDF; pore size:0.1 μm, 200 kDaMWCO

CSTR 10 HRT=12–48 hSRT=19–217 dOLR=0.03–0.11 kgCOD/m3/dMLSS=1–7.3 g/LTemp=25 °CpH=6.4±0.2

CODs=38–131pH=7.5

CODs=18–37 (55–69%)NH4

+–N=8.9–51.8NO3

−–Nb0.4 (0%)NO2

−–Nb0.4 (0%)pH=6.6±0.1

[89]

Domesticwastewater

L External Hollow fiber, MF,Pore size: 0.2 μm

CSTR 180 HRT=6 hMLSS=14–80 g/LOLR=2.16 kgCOD/m3/dTemp=25 °CFlux=7.5 LMH

CODt=540 CODs=65 (88%) [88]

Municipalwastewater

L Submerged Non-woven fabric, PET,pore size: 0.64 μm

UASB 12.9 HRT=2.6 hOLR=2.36 kgCOD/m3/dTemp=15–20 °CFlux=5 LMH

COD=259.5±343.8NH4

+–N=27.5±13.6TP=4.2±1.4

COD=77.5±29.5NH4

+–N=27.6±12.5TP=3.2±1.3

[48]

Domesticwastewater

L External UF membrane100 kDa MWCO

CSTR 50 HRT=15 hSRT>140 dMLVSS=0.5–10 g/LOLR=2.0 kgCOD/m3/dTemp=37 °CFlux=3.5–13 LMH

COD=685±46.4TOC=157±8.6BOD5=356±18.5Kj–N=156±7.8TP=11.5±0.6SS=380±9.3pH=7.2±0.2

COD=87.8±6.2 (88%)TOC=19±1BOD5=31.2±2.2 (90%)Kj–N=38.8±2TP=11±0.55SS=0pH=7.7±0.2

[90]

Municipalwastewater

L External Flat-sheet, CA,Pore size: 0.2 μm

CSTR 15 HRT=16.67 hMLSS=1.05–2.41 g/LTemp=35±2 °CFlux=80–450 LMH

COD=350–500NH4

+–N=15–21PO4

3−–P=3–4.5SS=1–1.5pH=6.5–7.8

CODb30 (>95%)TN=9–9.85 (15–20%)PO4

3−–Pb1 (81%)pH=6.9–7.3

[29]

Domesticwastewater

L External PTFE Teflonmembranepore size: 0.45 μm

CSTR 850 HRT=14.4 hOLR=0.8 kgCOD/m3/dTemp=22 °C

CODt=620–650 (637)TOC=180–230 (207)NH4

+–N=56–61 (58)Kj–N=70–78 (74)TP=10–12 (11)

TOC=17 (>90%)Kj–N=67TP=10

[92]

Municipalwastewater

L External PVDF; pore size: 0.1 μm,200 kDa MWCO

CSTR 10 HRT=48 hMLSS=1.01±0.29OLR=0.03±0.01 kgCOD/m3/dSRT=19 dTemp=32 °C

CODs=84±21NH4

+–N=27.3±13.5PO4

3−–P=6±2.3NO3

−–N=0.3±0.2TSS=120±60pH=7.5±0.1

CODs=25±12 (58±14%)NH4

+–N=8.9–51.8NO3

−–Nb0.4 (0%)NO2

−–Nb0.4 (0%)pH=6.6±0.1

[41]

a L=laboratory/bench scale.b PVDF=polyvinylidine fluoride, PET=polythylene terephthalate, CA=cellulose acetate, and PTFE=polytetrafluoroethylene.c CSTR=completely stirred tank reactor and UASB=upflow anaerobic sludge blanket.d The concentration unit is mg/L if not specified; CODs=soluble COD, and CODt=total COD.e The concentration unit is mg/L; and removal efficiency is presented in parentheses.


hydrolysis of solidmatters. The applied HRT, however, was significantlylower than the applied SRT with a range of 20–70.5 d (Table 6). Thesestudies confirmed the proposed advantage of AnMBR, which decouplesSRT from HRT, over conventional anaerobic digestion process. The ap-plied HRT appears to be efficient for hydrolysis and methanogenesisprocesses. It is evident from the study of Trzcinski and Stuckey [101]

who reported that no SS, soluble COD and VFA accumulation occurredinside AnMBR during treatment of municipal solid waste. The ap-plied OLR was usually higher than 1 kg COD/m3/d, and some caseshigher than 10 kg COD/m3/d, demonstrating the capacity of AnMBRto handle certain variation of OLR. The COD removal was generallyhigher than 90%.


AnMBRs have been used to treat landfill leachate and municipalsolid waste leachate. Landfill leachate is a high organic matter andammonium nitrogen strength wastewater formed as a result of per-colation of rain-water and moisture through waste in landfills. Thechemical composition of landfill leachate is dependent upon theage and maturity of the landfill site. The typical recent applicationsregarding AnMBR treatment are summarized in Table 7. It can beseen from Table 7, under selected operational conditions, high CODremoval of about 90% could be achieved. The applied OLR was gener-ally higher than 2.5 kg COD/m3/d. Marisa and Beal [103] reportedthat COD removal was 90.4% for an AnMBR and 21.5% for an anaero-bic filter (AF) when treated the same landfill leachate, indicatingmembrane separation significantly improved COD removal. Duringthese treatments, the inhibition of microbiological activity by landfillleachate was observed, and thus resulted in a reduced COD removal[104]. This effect, however, can be mitigated by using diluted landfillleachate as feed [104]. Also, the treatment efficiency can be im-proved by prolonged SRT and PAC addition [105].

3.2. Applications in biogas production and energy recovery

One notable advantage of the anaerobic process is the biogas recov-ery. Continuous biogas production could be observed in AnMBR systemsfor various wastewaters treatment. The observed methane yield ranged0.23–0.33 LCH4/g CODremoval has been reported [30,78,107–110], whichis generally lower than the theoretical yield (0.382 LCH4/g CODremoval

at 25 °C). The lower observed methane yield would be attributed tohigh methane solubility [111] and some inhibitors associated with an-aerobic process [112]. Lettinga et al. [113] observed more than 50%methane escape with treated effluent by UASB and attributed it to dilutenature of the sewage. Meanwhile, methane solubility is significantlyaffected by operational temperature. Methane is approximately 1.5times more soluble at 15 °C compared to 35 °C, for a typical biogasmethane content of 70%. Capturing dissolved methane is of great in-terest (particularly for dilute wastewater), since the loss of dissolvedmethane with the effluent would offer significant challenge for energyrecovery, and also cause greenhouse gas emission. Several processeshave been recently proposed for this purpose, including stripping ofAnMBReffluent through post-treatment aeration [114],methane recov-ery using a degassing membrane [115], and the use of a down-flowhanging sponge (DHS) reactor [116].

The composition of the biogas produced from AnMBR appears tobe: 70–90% methane, 3–15% carbon dioxide and 0–15% nitrogen[30,39,71,107–109]. The methane rich biogas can be used for digesterheating, electricity generation or even recycled for fuel production. Itwas reported that 2.02 kWh/kg CODRemoved can be produced from anAnMBR treating synthetic wastewater, which is approximately 7 timesmore than is required to operate the system [75]. In general, methanefermentation is a complex process divided up into four phases: hydroly-sis, acidogenesis, acetogenesis/dehydrogenation, andmethanation. Eachphase is carried out by different consortia of microorganisms whichplace different requirements on the environment [117]. Several factorssignificantly affect methane production. High temperature is known tobenefit the maximum specific growth and substrate utilization rates,and thus increase methane production. However, temperature changesor fluctuations were found to affect the biogas production negativelyin SAnMBR [118]. Furthermore, thermophilic processes are more sensi-tive to temperature fluctuations and require longer time to adapt to anew temperature. Meanwhile, the thermophilic process temperatureresults in a larger degree of imbalance and a higher risk for ammoniainhibition [119]. It's well-known that methane formation takes placewithin a relatively narrow pH interval, from about 6.5 to 8.5 with anoptimum interval between 7.0 and 8.0. The process is severely inhibitedif the pH decreases below 6.0 or rises above 8.5 [119]. Characteristics ofthe organic compounds also exert significant influences on methaneproduction. Organic wastes rich in carbohydrates, such as biowaste

and corn silage, can improve the biogas production and the proportionof CH4 [120].

3.3. Operational conditions

The main operational conditions related to AnMBR applicationsinclude hydrodynamic conditions, HRT, SRT, pH and temperature. ForAnMBRs with external configuration, employing high liquid cross-flowvelocity (CFV) along the membrane surface is a common operation toreduce the particle deposition over the membrane surface. However,high shear conditions have also been reported as detrimental for anaer-obic biomass activity and/or responsible for the physical interruption ofsyntrophic associations—a key factor in the anaerobic degradation oforganic matter [13]. Typically, CFV values of 2–3 m/s are sufficient toprevent the formation of reversible foulingwhile have no obvious effecton microbial activity in external configuration [69]. For submerged con-figuration, biogas sparging is the most common way to provide shearconditions [9,67,109,121,122]. However, to the best of our knowledge,no studies have assessed the effects of biogas sparging rate onmicrobialactivity or organic removal performance in AnMBRs.

It can be seen from Tables 3 to 7, the applied HRT in AnMBR variedfrom 2.6 h to 14 d, while the typical HRT for high strength wastewatertreatment and dilute wastewater treatment was 1–10 d and 0.25–2 d,respectively. Elongating HRT could generally improve pollutants remov-al, but only to a limited extent. For example, Hu and Stuckey [109] ob-served a marginal decrease in COD removal (approximately 5% overall)when they lowered the HRT from 48 h to 24, 12, 6, and 3 h during treat-ment of simulated dilute wastewater. SRT remains one of the mainoperational parameters determining both treatment performance andmembrane fouling. In contrast to UASB reactor, AnMBR enables com-pleted retention of biomass, and thus provides easier control of SRT.Trzcinski and Stuckey [105] investigated the performance of twoSAnMBRs treating municipal solid waste leachate at psychrophilic tem-perature with SRT of 300 and 30 d, respectively. It was found that longerSRT was associated with higher soluble COD removal [105]. In contrast,Baek et al. [89] found that the decrease in SRT from 213 to 40 d didn't af-fect treatment performance or membrane fouling. This suggests that therelationship between SRT and treatment performance or membranefouling is complex, and highly depends on the applied HRT and thefeed characteristics. In general, AnMBR operation with relatively longHRTs and SRTs was favorable, to enhance methane recovery, treatmentperformance and reduce sludge production [108].

Most AnMBR systems operate at near neutral pH since anaerobicdigestion takes place within pH 6.5–8.5 with an optimum interval be-tween 7.0 and 8.0 [119]. Such a pH range was usually achieved throughneutralization, which could require the excessive use of chemicals be-cause some streamshave extremepHvalues andhydrolysis, acidogenesisphases would decrease pH values. In this respect, equalization at a de-sired pH appears to be a prospective solution although related researchwas very limited in AnMBR systems. Anaerobic digestion is stronglyinfluenced by temperature and can be grouped under one of the follow-ing categories: psychrophilic (0–20 °C), mesophilic (20–42 °C) and ther-mophilic (42–75 °C) [86]. Higher temperatures are known to improvemethanogenesis, moreover, for several industries, including the pulpandpaper and textile industries, generate high temperaturewastewaters.Therefore, operation at thermophilic temperatures is of great interestbecause pre-cooling and post-heating used in the mesophilic treatmentfor subsequent reuse of treated effluent could be avoided. Several appli-cations operated at thermophilic temperatureswere available in the liter-ature [9,68,123,124]. However, a deterioration of membrane flux alwaysoccurred due to sludge deflocculation and EPS released caused by hightemperature [9]. Therefore, justification and selection of the operationaltemperature is important to achieve optimal performance. However, formost streams including municipal wastewater, operation at ambienttemperature or low temperature is essential for economical implemen-tation of AnMBRs treating them. Psychrophilic AnMBR treatment has

Table 6Summary of AnMBR performance for high-solids-content waste streams treatment.

Type ofwastewater


Type ofreactorc

Reactorvolume (L)

Operating condition Feedd Efficiencye Reference

Municipalsewage sludge

L Submerged Tubular stainlesssteel metalmembranepore size: 1.0 μm

CSTR+M 100 (Run2) HRT=2 dSRT=20 dSS=18–55 g/LTemp=35 °CpH=6 Flux=0.25 LMH

SS=5–30CODs=7

Most favorable fermentationefficiency was attained

[21]

Waste activatedsludge

L External Hollow fiber PEmembranepore size: 0.4 μm

UASB+M 8 HRT=6 dSRT=80 dTemp=37 °C

– VS destruction>52.1% [44]

Municipal(solid)waste

L Submerged Cylindricalwovennylon mesh;pore size: 30, 40,and 140 μm

CSTR+M HRT=1.5 d;SRT=20 dOLR=3.75 kgVS/m3/d

– [47]

Municipal solidwaste

L Submerged Kubota PE flatsheetmembrane;Pore size: 0.4 μm

CSTR/CSTR+M

10/3 HRT=–/1.6–2.3 dTemp=35±1 °CFlux=0.5–0.8 LMH

COD=–/4–26 InitialTSS=40/3.31 g/L

COD=4000–26,000/400–600(>90%)

[43]

Municipalsewage sludge

P External Vibrating unit;Teflon, UF;Pore size:0.05 μm

CSTR+M 550 HRT=1.7–11.8 dSRT=70.5 dTemp=35 °CTSS=1.8%Flux=60,7–83.3 LMH

TSS=0.6% TSS=0; Average TS and VSreductions were 51% and 59%,respectively, by the digester

[36]

Wastewaterscontainingsuspendedsolids

L Submerged Tubular PSF MFmembrane

CSTR+M 3.8 TSS=40 g/LOLR=10 kgCOD/m3/dTemp=30 °CFluxb4 LMH

COD=10 COD=150–200 [26]

Slaughterhousewastewater

L External MF 100 kDaMWCO

CSTR+M 50 HRT=1.66 dMLVSSb10 g/LOLR=8.23±2.5 kgCOD/m3/dTemp=37 °CFluxb3 LMH

CODt=10.174±3.31pH=7.53–7.7

CODs=338±60(94±2.12%)

[102]

Slaughterhousewastewater

L External MF 100 kDaMWCO

FBR/CSTR+M

25/50 HRT=1.25 dMLVSS=8.257–10.1 g/LOLR=12.7±1.71 kgCOD/m3/dTemp=37 °CFluxb3 LMH

CODt=10.58±0.99pH=7.53–7.7

CODs=196±4(98.75±0.44%)

[102]

Swine manure L External Tubular PES UFmembrane;20 kDa MWCO

CSTR+M 6 (Run 1) HRT=6 d;pH=7.5OLR=1 kgVS/m3/dTemp=37±1 °CFluxb0.3 LMH

Biomassconcentration=6 gVS/L

CODs=200–250 (86%),CODt=– (96%)

[42]

Sand-separateddairy manure

L External Tubular PVDFmembrane;Pore size:0.03 μm

CSTR/CSTR+M

100/100 HRT=9/9 d;SRT=28 dMesophilic conditionOLR=3.3/2.4 kgVS/m3/d

COD=44.9±12.1/31.8±6.56TS=4.54±0.69%/3.36±0.64%NH4

+–N=1.24±0.47/1.3±0.09TP=0.34±0.03/0.32±0.08

COD=3440±700(92±1.8%)TS=800±150%(81.7±5.3%)NH4

+–N=1330±90(−16.6±31.9%)TP=14±5 (95.8±1.5%)

[40]

a L=laboratory/bench scale and P=pilot scale.b PVDF=polyvinylidine fluoride, PE=polythylene, PSF=polysulfone, and PES=polyethersulfone.c CSTR=completely stirred tank reactor, UASB=upflow anaerobic sludge blanket and FBR=fixed bed reactor.d The concentration unit is g/L if not specified; – indicates value not reported; CODs=soluble COD, and CODt=total COD.e The concentration unit is mg/L and removal efficiency is presented in parentheses.


recently drawn significant attentions [105,110]. It was found that bothpsychrophilic and mesophilic treatment achieved comparable COD re-moval efficiency close to 90%, although the former corresponded to a littlehighermembrane fouling rate due to volatile fatty acid (VFAs) accumula-tion [110]. This result highlights the possible role of membrane filtrationin performance stability across temperature fluctuations. In order towidely apply the AnMBR technology, one key challenge is to overcomethe problems caused by the local climate change conditions within ap-proximately 0 to 25 °C. However, no studies have assessed AnMBR treat-ment performance of psychrophiles at elevated temperatures.

3.4. Applicability of AnMBRs

In current review, it is proposed that wastewater can be conceptu-alized as having three axes, including an x-axis (concentration of theconstituents), a y-axis (particulate nature of the constituents), and az-axis (extreme conditions, e.g. extreme pH, temperature, salinity)which represent the principle characteristics of wastewater (Fig.1).Fig.1 classifies wastewaters into 8 zones. For example, municipalwastewater characterized by low organic strength, low particulatecontent and less extreme properties, will fall in Zone VII. Pulp and

Table 7Summary of AnMBR performance for leachate treatment.

Type ofwastewater


Type ofreactorc

Reactor volume(L)

Operatingcondition

Feedd Efficiencye Reference

Landfillleachate

L External UF 100 kDaMWCO

CSTR+M 50 (Run 3) HRT=7 d pH=7.5OLR=6.27±0.78 kg COD/m3/dMLVSSb3 g/LTemp=37 °C

COD=41±3.14

COD=3.77±0.34(90.7±1.1%)

[106]

Sanitarylandfillleachate

L External Ceramic tubularmembranepore size: 0.2 μm

CSTR+M 44 HRT=2.04 dTemp=34–36 °COLR=5.07±2.90 kg COD/m3/d

– COD=– (90.4%)turbidity=2.0±2.0NTU(90.3%)

[103]

Diluted landfillleachate

L Submerged Capillary UFmembranePore size: 0.1 μm

CSTR+M 29 HRT=2 dpH=8.18OLR=2.5 kg COD/m3/dMLSS=10 g/LTemp=35 °C

COD=5NH4

+–N=0.382pH=8.03

COD=0.417 (90%),NH4

+–N=0.206[104]

Municipalsolidwasteleachate

L Submerged PE flat sheetmembranePore size: 0.4 μm

CSTR+M 3 HRT=1.5 d; SRT=30 dOLR=8 kg COD/m3/dpH=7.3Temp=35 °C

– COD=1.0 (79–95%), [105]

Municipalsolidwasteleachate

L Submerged PE flat sheetmembranePore size: 0.4 μm

CSTR+M 3 HRT=1.1 d;SRT=300 dOLR=11.7 kg COD/m3/dpH=7.3Temp=35 °C

– COD=– (90%) [105]

a L=laboratory/bench scale and P=pilot scale.b PE=polythylene.c CSTR=completely stirred tank reactor.d The concentration unit is g/L if not specified and – indicates value not reported.e The concentration unit is g/L and removal efficiency is presented in parentheses.


paper industrial wastewater will fall in Zone IV due to that it is char-acterized by high organic strength, low particulate content and ex-treme properties (e.g. high temperature).

AnMBRs appeared to be suitable to treat all types ofwastewaters ex-cept forwastewaters falling in ZoneVIII which are characterized by highorganic strength, low particulate content and favorable properties.Wastewaters falling in Zone VIII are currently treated effectively withvarious HRARs, such as UASB and EGSB. Biofilm and granule formationin HRARs would enable to decouple SRT from HRT, allowing for highOLR and organic removal with a minimum of SS in the effluent. Com-pared to AnMBRs, HRARs could achieve comparable or a little worseeffluent quality, while their capital and operational costs remain rela-tive low due to no membrane used. The opportunity for AnMBRs to beapplied to these wastewaters appears to only exist when very low SSconcentration in effluent and/or short start up period are required.

Low organic strength will favor AnMBRs treatment as compared toHRARs. Biomass loss and bad effluent quality are two major problemsassociated with HRARs treating low organic strength wastewaters.AnMBRs can solve these problems because membrane totally retainsbiomass and enhanced effluent quality achieved. AnMBRs treatinglow organic strength wastewaters have recently drawn considerableattention [8,30,109,125]. Increase in particulate content in wastewaterwill increase the applicability of AnMBRs. Retention of particulates inHRARs would be very problematic, and long SRT and HRT were usuallyrequired for sufficient hydrolysis. Thiswill significantly increase the cap-ital costs. Complete retention of particulates can be achieved in AnMBRs,which may allow greater treatment efficiency by allowing more com-plete hydrolysis of slowly degraded compounds. Application of AnMBRsis more likely restricted to conditions or applications where granularsludge technology may or will encounter problems. This likely is thecase when extreme conditions prevail, such as high temperatures andhigh salinity, or wastewaters with refractory and/or toxic compounds,since biofilm and granule formation can be severely affected. Followingthe current trend of increasing water use efficiency and closing

industrial process water cycles, these extreme conditions are likely tobecome more common in the future [126]. It is expected that AnMBRswill get more opportunities in these wastewaters treatment.

4. Membrane fouling issues

Membrane fouling remains the critical obstacle limiting the morewidespread application of AnMBR inwastewater treatment. Membranefouling could decrease system productivity, cause frequent cleaningwhich might reduce the membrane lifespan and result in higherreplacement costs, and increase the energy requirement for sludgerecirculation or gas scouring. Membrane fouling results from interac-tion between the membrane material and the components of sludgesuspension. Though the membrane used in aerobic MBR can be gener-ally used in AnMBR system, the sludge suspension in AnMBR systemis significantly different from that in aerobic compartment, presentingcertain unique impacts on membrane fouling characteristics. A set oftechniques or approaches are now available to characterize membranefouling [127], which allows for better understanding ofmembrane foul-ing in AnMBR system. To date, there have been a considerable numberof published papers on AnMBR system, perusal of the literature showsthat there is a lack of a comprehensive review regarding membranefouling specific for AnMBR system.

4.1. Membrane fouling classification

Membrane fouling can be traditionally classified into reversible andirreversible fouling based on the cleaning practice, although their defi-nitions were not consistent in the literatures. Here, we adopt the classi-fication proposed by Meng et al. who further defined reversible foulinginto removable fouling and irremovable fouling. Accordingly, remov-able fouling refers to fouling that can be removed by physical meanssuch as backflushing or relaxation under cross flow conditions, whileirremovable fouling refers to fouling needed to be removed by chemical

Zone I

Extrem

e Properties

Zone V

Zone VIIIZone VII

Z ne

Zone IVZone III Z ne II

Fvor

eP

rerties

Fig. 1. Conceptualization of wastewaters according to their principle characteristics.


cleaning. The irreversible fouling is a permanent fouling which cannotbe eliminated by any cleaning approaches. In general, removable foul-ing occurs due to loose external deposition of material. In contract, irre-movable fouling is caused by the pore blocking and strongly attachedfoulants during membrane filtration. Formation of a strong matrix offouling layer with the solute during a continuous filtration processwill result in removable fouling being transformed into an irremovablefouling layer. Considering the nature and the causes of irremovablefouling, many efforts have been performed to investigate cake layer.During the long-term operation of a SAnMBR, Jeison and van Lier[122], and Gao et al. [128] observed that cake formation and consolida-tion,which could not be removed by the back-flush cycles or relaxation,provide the dominantmechanism of membrane fouling as compared tointernal pore fouling. Meanwhile, Di Bella et al. [129] reported that thecake layer formed in an aerobic MBR had a mainly removable nature.Although cake formation is a complex process and involved a lot ofinfluencing factors, it might be concluded that, on average, cake layerformed in AnMBR has relatively lower removability than that in aerobiccompartment due to the different sludge properties.

Membrane fouling can also be classified into biological, organicand inorganic fouling in viewpoint of the foulant components [3,46].Biological fouling is specifically related to the interaction of biomasswith the membrane. Membrane fouling appears to start from poreclogging caused by cell debris and colloidal particles. Passive adsorp-tion of colloids and organics has been observed even for zero-flux op-eration, before the biomass deposition initiates [129]. Gao et al. [128]reported that about 65% of the particles based on number in the topcake layer in a SAnMBR had a size smaller than 0.3 μm that was thesame to the pore size of the used membrane. These particles/flocswould penetrate into and block the membrane pores easily. Biofoul-ing also includes the accumulation and adsorption of extracellularpolymeric substances (EPS) and soluble microbial products (SMP)on membrane and pore surfaces as these substances were biologicallysecreted. Meanwhile, some studies have been performed to investi-gate the microbial community and its role in membrane fouling inAnMBRs. Gao et al. [130] and Lin et al. [131] found that there weresignificant differences in microbial communities between sludge onmembrane surfaces and in bulk sludge in an external and submergedAnMBR, respectively, suggesting that some bacteria selectively ad-hered and grew on the membrane surface.

Organic and inorganic fouling usually respectively refers to macro-molecular species (biopolymers) and scalants. Lin et al. [9] also observed

the supernatant CODwas consistently higher than the effluent COD for aSAnMBR. The significantly higher content of organics in the supernatantwas believed to be biopolymer matters, which may act as a “glue”, facil-itating a cake layer formation. Analysis through Fourier transform infra-red (FTIR) spectroscopy and confocal laser scanningmicroscopy (CLSM)demonstrated that the foulants on the membrane surface in SAnMBRwere rich in proteins and polysaccharides [9,128,132], indicating thatorganic fouling was originally caused by SMP or EPS. As for inorganicfouling, struvite (MgNH4PO4·6H2O) appeared to one of the main inor-ganic foulants identified earliest in AnMBR systems [13,28,133,134].Other inorganic foulants can include K2NH4PO4 and CaCO3 [90]. Precip-itation of inorganic foulantsmuch depends on the presence of cations inthe influent and sludge suspension, which is the origin of inorganicelements in cake layer. Through charge neutralization and bridging ef-fect, metal clusters and metal ions in the influent could be caught bythe flocs or biopolymers and then enhanced filtration resistance [135].Lin et al. [70] reported that the cake layer in a SAnMBR was formed byorganic substances and inorganic elements such as Ca (4.45% dryweightcontent), Mg (1.94%), Al (1.72%), Si (1.46%), K (0.15%), etc. Herrera-Robledo et al. [32] found that cake sludge in AnMBR was mainly com-posed of volatile solids (85%) and the rest was related tomineralmatter.Similar results have also been reported by other researchers [33,48].

It should be borne in mind that biological, organic and inorganicfouling take place simultaneously, and the interaction of them usuallyincreases filtration resistance. For example, Choo and Lee [13] reportedthat deposition of themicrobial cells togetherwith struvite played a sig-nificant role in the formation of the strongly attached cake layer limitingmembrane permeability. From one point of view, membrane fouling isgenerally characterized by initial pore clogging followed by biocake for-mation and consolidation regardless of aerobic and anaerobic MBRs.However, the forms and significance of membrane fouling in AnMBRwould be of some differences. For instances, Gao et al. [128] foundthat cake thickness in a SAnMBR could be 1900–2100 μm, which wasmuch higher than 20–200 μm reported on aerobic MBR systems[136,137]. For the same membrane, the maximum sustainable mem-brane flux was found to be 11 and 25–30 LMH for SAnMBR [30] andaerobic SMBR [138] for municipal wastewater treatment, respectively.Also, considering the relatively high concentration of carbonate andbiocarbonate [139], and the production of high ammonia and phos-phate concentrations in anaerobic digestion, AnMBRs may be moresusceptible to inorganic fouling than aerobic MBRs. The unique charac-teristics of membrane fouling suggest that more attention should bepaid on its control in AnMBR.

4.2. Membrane fouling mechanisms

Based on their relative contributions of foulant components to thetotal membrane fouling, several membrane fouling mechanisms, in-cluding pore plugging/clogging by colloidal particles, adsorption ofsoluble compounds and biofouling, deposition of solids as a cake layer,cake layer consolidation [122] and the spatial and temporal changes ofthe foulant composition [140] during the long-term operation, havebeen proposed.

The current trend in AnMBR design is to operate at constant flux.When operated in this mode, a three-stage trans-membrane pressure(TMP) profile characterized as an initially short term rapid TMP rise(stage 1) followed by extended slow TMP rise period (stage 2) and atransition to a rapid TMP rise (stage 3), which typically occurred inaerobic MBR operation [141], can also be observed (Fig. 2a) in AnMBRs[9,27,70,142,143]. The possible mechanisms for each stage are illustrat-ed in Fig. 2b, c, and d according to previous studies [133,142]. Undersuction drag and gas scouring in SMBR, there are two opposite forcesthat control the deposition of sludge components on membranesurface: permeation drag, which is generated by permeate flux, in-creased with operation TMP, and back transport, consisted of Browniandiffusion, inertial lift and shear induced diffusion [144]. Initially, the


colloids and soluble products can be readily deposited onto the mem-brane surfaces by permeation drag, and not readily detached by shearforce due to its low back transport velocity [145]. Their higher deposi-tion tendency over large flocs has been verified in SAnMBR systems[9]. These colloids and soluble products usually have size lower thanthe pore size of the used membrane, and would readily penetrate intoand block the membrane pores, and therefore caused significant mem-brane fouling, which would be responsible for the first TMP jump inFig. 2a (Fig. 2b). The deposited colloids and soluble products were alsoconsidered to play the role of conditioning themembrane surface, facil-itating followed cake formation [11]. The gradient developing sludgecake would prevent the further penetration and blocking of membranepores by the colloids and soluble products, and itself corresponds to theslow TMP increase (stage 2, Fig. 2c). To date, the interpretation to thesecond TMP jump is still debated. The most popular interpretation islocal flux theory which was firstly introduced in AnMBR system byCho and Fane [142]. They attributed the second TMP jump to thechanges in the local flux due to uneven distribution of foulants andEPS causing local flux to be higher than the critical flux [142]. However,even for membrane with relatively uniform distribution of foulants, thesecond TMP jumpwas also observed in a SAnMBR [9,146]. Hwang et al.[140] recently reported that the sudden jump of TMP was closely relat-ed to the sudden increase in the concentration of EPS at the bottom ofcake layer, which might be attributed to the death of bacteria in theinner of cake layer. As membrane fouling is a really complex process,combination of the two explanations appears to be more extensiveand reasonable than single interpretation alone.

Sludge cake consolidation (compression) is inevitable as TMP in-creases. After cakewas formed onmembrane surface, cake consolidationis a kind of sludge dewatering process. Activated sludge ismade up ofmi-crobial organisms and colonies, embedded in amatrix of EPS [147]whichcarry charged functional groups, including carboxyl, hydroxyl and phos-phoric groups, leading to the presence of large concentrations ofcounter-ions within the matrix of EPS for reasons of electro-neutrality[148]. These counter-ions closely associated with the matrix of EPS incake layerwill not readily go through themembrane, thus, the differencein salt concentration between two sides of the membrane will result inan osmotic gradient. Chen et al. [149] recently reported that osmoticpressure accounted for the largest fraction of total operation pressureduring cake layer filtration, indicating that osmotic pressure generatedby the retained ions was one of the major mechanisms responsible formembrane fouling problem in SAnMBR once a cake layer was formed.

4.3. Parameters affecting membrane fouling

Membrane fouling results from the interaction between membraneand sludge suspension. In this regard, all the parameters related tomem-brane and sludge suspension would have effects on membrane fouling.These parameters can be generally classified into four categories: feedcharacteristics, broth characteristics, membrane characteristics and op-erational conditions. The effects of these parameters on membrane foul-ing are summarized in Table 8 mostly based on the recent literaturerelated to AnMBR. Among them, some parameters, such as SMP, EPS,particle size distribution (PSD) and hydrodynamic conditions, have di-rect effects on membrane fouling, and therefore were considered as themajor parameters affecting membrane fouling. In contrast, some others,such as HRT, OLR, SRT and pH indirectly affect membrane foulingthrough the change in the broth characteristics. A comprehensive assess-ment of the major parameters and indirect affecting parameters inAnMBR appears to be not necessary, since membrane fouling mecha-nisms are generally similar in MBR systems, and previous reviews ofMBR fouling have warranted separate presentations [11,46]. However,it should be noted that, for AnMBR treatment, the changewill be the rel-ative importance of these parameters under specific conditions. Table 9compares some facets of these major parameters in aerobic MBR andAnMBR. It is expected that the higher MLSS, OLR, residual COD and

SMP production in AnMBR will cause more serious membrane foul-ing. Moreover, the extreme conditions (pH and temperature) relatedto AnMBR treatment will induce decreased PSD of sludge liquor,which in turn negatively affect membrane fouling. For instance,under similar operational conditions, Martin-Garcia et al. [150]found that SMP in AnMBR supernatant was 500% higher than thatin aerobic MBR supernatant. Considering different broth characteris-tics and operational conditions, it may be not surprising to concludethat, on average, AnMBR treatment would result in more serious mem-brane fouling problems. This comparison suggests that more attentionshould be paid on membrane fouling control in AnMBRs.

4.4. Membrane fouling control

The purpose of membrane fouling study is to develop strategiesfor membrane fouling control and membrane cleaning. Based on theparameters affecting membrane fouling, these strategies in AnMBRsystems can be classified into five groups: (1) pretreatment of feed,(2) optimization of operational conditions, (3) modifying activatedsludge, (4)modification of membrane and optimal design of membranemodule, and (5) membrane cleaning.

4.4.1. Pretreatment of feedFeed characteristicsmay exert significant impacts onmembrane foul-

ing. Some industrial streams contain trash which can plug the coarsebubble diffusers used to scour the membranes. The extreme pH condi-tions in some industrial wastewaters not only damage biologic perfor-mance, but also affect membrane permeability and lifespan. It has beenreported that cake layer on membrane surface was rich of elementsMg, Al, Ca, Si, and Fe [70]. These components apparently originatedfrom the inorganicmatters in the feed. Interaction of biopolymermattersand these elements were reported to have significant impacts on theformation and compactness of the cake layer [13,70]. Excess quantitiesof thesematerials should be removed throughwastewater pretreatmentprograms (i.e., filtration [92], pH adjustment [78], establishment of localwastewater limits). Kim et al. [28] used a dialyzer/zeolite (D/Z) unitto selectively remove NH4

+ in the influent, in which substantial NH4+

removal (in excess of 90%)was achieved, leading to the significant reduc-tion in struvite precipitation on the ceramic membrane in the AnMBR.

4.4.2. Optimization of operational conditionsThemain operational parameters include hydrodynamic conditions,

flux, HRT, SRT, biomass concentration, pH and temperature. Increasingthe gas scouring intensity and time in SAnMBRs and the flow velocityof mixed liquor in sidestream AnMBRs could certainly achieve betterhydrodynamic conditions for membrane fouling control. However, itcould also disrupt sludge flocs, producing small size particles and re-leasing more EPS which negatively impact membrane fouling [9,45].Jeison et al. [45] introduced the concept of “shear rate dilemma” to de-scribe the dual effects of shear during AnMBR operation. There exists apractical limit above which only a minor benefit is provided. Pilottesting is required to find optimal hydraulic conditions. A well knownstrategy for membrane fouling control is to operate membrane at sus-tainable flux. Detailed discussion on critical flux and sustainable fluxcan be found elsewhere [163]. Other above mentioned parameterswill directly affect broth properties. In this regard, control strategiesshould focus onmodifying broth properties by adjusting these parame-ters. The relationship between these parameters and broth propertiescan refer to Table 8.

4.4.3. Modifying broth propertiesAddition of the additives, such as adsorbent agents, coagulants, car-

riers, suspensible particles and other chemical agents, can modify theproperties of the broth inAnMBRs. Suited additives for foulingmitigationcan act through a number of different phenomena such as adsorptionof SMP, coagulation, cross-linking between flocs, and a combination of

Sludge flocColloidSolute

Filtration time

TM

P

Stage 1

Stage 2

Stage 3

Permeat Drag

Shear Force

Membrane

Permeat Drag

Shear Force

Membrane

Permeat Drag

Shear Force

Membrane

(a) (b) Stage 1

(c) Stage 2 (d) Stage 3

Fig. 2. Schematic illustration of the three-stage TMP profile and its fouling mechanisms.


these [164]. Meanwhile, some novel measures have been developed forthe optimization of mixed liquor in recent years, providing variousoptions for membrane fouling control in AnMBRs.

Powdered activated carbon (PAC) is the most widely used “fluxenhancers” in MBRs. The first study testing the effect of PAC on foulingmitigation in AnMBR appeared to be reported on 1999 [165], and itwas found that both the fouling and cake layer resistances decreasedcontinuously with increasing the PAC dose up to 5 g/L. The enhancedmembrane performance in AnMBR due to PAC addition has been con-firmed by many studies [23,121,134,165]. The mechanism of PAC forfoulingmitigationwas supposed to be adsorption of the solutes and col-loids in the supernatant [121], and enlarged floc size due to incorpora-tion of PAC to the bioflocs [166]. However, an overdose of PAC couldincrease membrane fouling because excess PAC itself could be a foulant[121,167]. Other adsorbent agents like zeolite, bentonite, vermiculiteand Moringa oleifera were also used to mitigate membrane foulingin AnMBRs [168,169]. These additives have high adsorption and ion-exchange capacity, and therefore are capable to reduce soluble organicsand NH4

+ in the supernatant. Improved effluent quality and membraneperformancewere usually observed in these studies. To date, coagulantsincluding aluminum sulfate, ferric chloride, polyaluminum chloride(PACl), polyferric sulfate (PFS), polyacrylamide (PAM) and chitosanhave been tested in aerobic MBRs [170–172], and alleviated membranefouling due to increased floc size and decreased soluble organics in su-pernatant were generally observed. Addition of chemical coagulants tothewastewatermay cause side effects by producing by-products and/orincreasing the volume of sludge in the reactor [173]. An alternativetechnology for creating coagulation inside the system, suggested bythe Bani-Melhem and Elektorowicz [174,175], is to introduce electroki-netic processes into the MBR. The design of electro-kinetic process wasbased on applying an intermittent direct current (DC)field between im-mersed circular perforated electrodes around an immersed membranefiltrationmodule. Such a systemnot only significantly reduced the foul-ing rate, but also enhanced the removal of COD and PO4

3−–P up to 96%and 98%, respectively [175]. These studies demonstrated the great po-tential of utilization of coagulants in AnMBRs.

Recently, Chae et al. [176] investigated potential use of fullerene C60nanoparticle addition for membrane biofouling control. It was foundthat C60 significantly impeded bacterial surface attachment.Magnesiumor titanium oxide and copper-based nanoparticles [176] could be otherpotential additives for fouling mitigation. This study provided a novel

option for membrane fouling control in AnMBRs. Another notablenovel measure is the use of ozone for sludge modification. It was foundthat ozonation enlarged suspended flocs by reducing zeta-potentialand increasing hydrophobicity, thus enhancingflocculability of the parti-cles in the mixed liquor, and mitigate membrane fouling [177,178]. Forthis measure, an optimal dosage is critical because overdosing wouldbreak the flocs and release colloidal and soluble organics, and thereforeexacerbate fouling [177]. More recently, a granular AnMBR seeded withgranular sludge from a UASB was developed by Martin-Garcia et al.[150]. As compared to parallel operated flocculated AnMBR, colloidsand SMP in the supernatant were significantly reduced in the granularAnMBR. This study showed that development of granular sludge inAnMBR could increase filtration ability of broth supernatant, and presentan effective strategy for membrane fouling control.

4.4.4. Membrane optimizationSurfacemodification for hydrophilic improvement of membrane is a

common strategy for membrane fouling control since the most salientproperty of the membrane materials is their surface properties. Surfacemodification with aims to implant polar organic functional groups ontothe membrane surface could be achieved by means of plasma treat-ment, surface grafting, surface coating and surface blending, etc. Plasmatreatment appears to be an efficient technique to create hydrophilicfunctional groups on the membrane surface. It was found that mem-brane hydrophilicity significantly increased after NH3 and CO2 plasmatreatments, and new membranes presented better filtration perfor-mances and flux recovery than those of unmodified membranes[179,180]. To date, plasmas including air, O2, N2, CO2, H2O, and NH3

plasmas, have been explored [179–183]. The unique advantage of plas-ma treatment is that the surface properties and biocompatibility can beenhanced selectively while the bulk attributes of the materials remainunchanged. Whereas, the complex chemical reactions and the largenumber factors affecting treatment efficiency involved in plasma treat-mentmake it difficult to extend such technology on large-scale. Surfacegraft polymerization is another attractive method to improve mem-brane hydrophilicity. By performing UV photo-induced graft polymeri-zation of acrylic acid and acrylamide on a PP MFmembrane surface, Yuet al. [184] observed the decreased water-contact angle and increasedzeta potential (absolute value) with increased grafting degree. A PPmembrane modified by ozone treatment followed by graft polymeriza-tion with 2-hydroxy-ethyl methacrylate (HEMA) has been applied in

Table 8Description of the effects of fouling parameters on membrane fouling in AnMBRs.

Fouling parameters Description of the effects on membrane fouling Wastewater Ref.

Operational conditionsHRT HRT↓→biomass concentration↑, PN/PS in SMP↑→dTMP/dt↑ Synthetic low-strength wastewater [125]

HRT↓→EPS↑, SMP↑→cake resistance↑ Acidified wastewater [25]HRT↓→biopolymers↑, floc size↓→specific cake resistance↑ Synthetic municipal wastewater [151]

OLR OLR↑→VFA concentration↑, predominant VFA type changed Synthetic coke wastewater [68]SRT SRT↑→sludge activity↓, SMP↑→dTMP/dt↑ Synthetic low-strength wastewater [125]

SRT↑→MLVSS↑, floc size↓→ irreversible fouling↑ Synthetic low-strength wastewater [67]Hydrodynamic conditions Gas sparging rate↑→critical flux↑ Kraft evaporator condensate [71]

Gas sparging time↓→TMP↑ Saline sewage [23]CFV↑→shear force↑, floc size↓→critical flux firstly↑ then↓ Acidified synthetic wastewater [45]Gas sparging was ineffective in increasing the critical flux Acidified wastewater [152]CFV↑→SMP↑, floc size↓→flux↓ Diluted anaerobic sludge [153]

Permeate flux Permeate flux↑→ long-term operation period↓ Swine wastewater [154]Permeate flux↑→cake formation rate↑ Kraft evaporator condensate [146]Permeate flux↑→ fouling rate↑ Domestic wastewater [155]

Temperature Temperature↓→CODsup↑→stable flux↓ Municipal solid waste leachate [105]Temperature↑→CODsup↑, floc size↓, PN/PS of EPS↑→filtration resistance↑ Kraft evaporator condensate [9]Temperature↑→viscosity↓, COD removal↑→flux↑ Food wastewater [10]

Biomass characteristicsMLSS MLSS↑→ initial and stabilized flux↓, optimal MLSS=15–18 g/L Diluted anaerobic sludge [153]

MLSS↑→TMP↑ Food industry wastewater [156]MLSS↓→solids deposition rate↓ Dilute municipal wastewater [89]

PSD Amount of small flocs↑→filtration resistance↑ Kraft evaporator condensate [9]Floc size↓→specific cake resistance↑ Synthetic municipal wastewater [151]D0.1↑→cake formation rate↓ Kraft evaporator condensate [146]

SMP SMP↑→filtration resistance↑ Kraft evaporator condensate [9]High-MW protein and carbohydrate material↑→ internal fouling↑ Low-strength synthetic feed [8]Low flux was attributed to high amounts of SMP Medium strength wastewater [157]

EPS PN/PS ratio↑→ fouling rate↓ TMP whitewater [39]EPS↑→cake resistance↑ Acidified wastewater [25]EPS the foulant layer contributed to membrane fouling Particulate artificial sewage [130]

Microbial community Some bacteria play a pioneering role in cake formation TMP whitewater [131]Relative abundance of bacteria was different in cake layer and suspension Artificial sewage [130]

Membrane characteristicsMWCO↑, surface roughness↑→flux decline↑, recoverable flux rate↓ Food wastewater [10]Pore size↑→attainable flux↓ Synthetic wastewater [158]Fouling of PEI membrane was faster than PVDF membrane coated with PEBAX Artificial sewage [130]


an AnMBR, and the results showed that the membrane permeabilitywas significantly enhanced [24]. However, there are the two majorproblems remained: the difficulty in obtaining optimal value of graftingchain length and grafting density for membrane permeability andmembrane antifouling characteristics, and the high costs of employinghigh-energy induced methods, such as UV irradiation [184], gammairradiation [185], and chemical reaction [24]. Surface coating via ad-sorption of surfactants was also explored. A significant example is theinvestigation of Kochan et al. [186], where different UF flat-sheetmembranes, PVDF, PES, PSF and cellulose acetate (CA) were coated bybranched poly(ethyleneimine) (PEI), poly(diallyldimethylammoniumchloride) (PDADMAC) and poly(allylamine chloride) (PAH) and filtratedwith sludge supernatant, and it is found that coating led to lower foulingrates during filtration. The disadvantages of this measure would be thelow physical tolerance (to, e.g., desorption, cross-flow and gas scouring)and the chemical stability of the coating layer under MBR conditions. Toovercome above disadvantages, a self-assembly techniquewas employedto create thin film composite nanofiltration membranes (TFC NF), whichwas achieved by coating commercial PVDF UF membrane with the am-phiphilic graft copolymer PVDF-graft-polyoxyethylene methacrylated(PVDF-g-POEM) [187]. The new TFC NF membranes exhibited no irre-movable fouling in 10 d dead-end filtration of model organic foulants(bovine serum albumin (BSA), sodium alginate and humic acid) at con-centrations of 1000 mg/L and above. Meanwhile, TiO2 embedded poly-meric membranes prepared by a self-assembly process have recentlydrawn considerable attention. When applied for activated sludge filtra-tion, it was found that adsorbed foulants on the TiO2 embedded

membrane surface were more readily dislodged by shear force thanthose on neat polymeric membranes due to the increased hydrophilicityof the membrane [188,189]. In general, the above modified membranescan be used in aerobic systems as well as anaerobic systems.

4.4.5. Membrane cleaningMembrane fouling can never be completely avoided, while the

fouled membranes can be regenerated by physical, chemical, and bio-logical schemes. Physical cleaning techniques for MBRs include mainlymembrane relaxation and membrane backflushing. Detailed discussionon these conventional physical cleaning measures can be found in theprevious review paper [11]. In recent years, a novel on-line physicalcleaning method, ultrasonication, has been developed and extensivelyinvestigated in MBRs, especially in AnMBRs [44,66,190,191]. Wen et al.[191] showed that ultrasound can effectively control cake formationon the membrane surface. The mechanism of ultrasonication for mem-brane fouling control was considered to be cavitation and acousticstreaming induced by ultrasonic waves preventing the cake formationand enhancing membrane filtration rates [192]. Meanwhile, it wasfound that ultrasonic irradiation could negatively affect anaerobic bacte-rial activity [190] and cause membrane damage [191]. These effects,however, can be significantly reduced by properly selecting ultrasonicintensity and working time and keeping a certain thickness of cakelayer on the membrane surface [191].

When the above-mentioned cleaning methods are not effectiveenough to reduce the fouling to an acceptable level, it is necessary toclean the membranes chemically. Many chemical cleaning agents, such

Table 9Some facets of membrane fouling propensity in aerobic MBRs and AnMBRs.

Parameters Aerobic MBR AnMBR Potential effects on membrane fouling

MLSS Lower MLSS maintained in bioreactor Similar or higher MLSS maintained in bioreactor MLSS is positively correlated to membranefouling [156,159]

OLR Low High Filtration resistance increases with organicloading [160]

SMP Depends Flocculated AnMBR supernatant was characterized by a SMPconcentration ca. 500% higher than the aerobic MBR [150]

High SMP content results in serious mem-brane fouling [161]Soluble SMP amount in the mixed liquor wasthe most important property influencing thefouling propensity of sludge [162]

PSD More often, aerobic MBRs were used formunicipal wastewater treatment. Mild nature ofmunicipal wastewater favored flocs growth.

More often, AnMBRs were applied for industrial wastewatertreatment. Extreme conditions of industrial wastewater werefrequently encountered, which would cause dispersed flocs

Flocs size significantly affected cakeformation, filtration resistance [131,145]


as sodium hypochlorite (NaClO), hydrochloric acid (HCl), nitric acid,citric acid, sodium hydroxide (NaOH) and EDTA, have been frequentlyemployed for membrane cleaning in AnMBRs [30,33,122,132]. Efficientchemical cleaning requires the selection of cleaning agents that targetdominant compounds responsible for fouling and that do not adverselyaffect the membrane itself. In general, oxidizing and alkaline agents,such as NaClO andNaOH, are used to remove themicroorganisms and or-ganic foulants. Acidic agents are effective in breaking metal-associatedstructures including metal organic foulant complexation and inorganicscales. Coordination agents like citric acid and EDTA can removemetallicfoulants aswell, due to their outstanding binding abilitywithmetal ions.It is evident that a combination of cleaning agents, such as NaClO andNaOH, is more efficient than single-agent methods [193]. The typicalcleaning protocol used in AnMBRs comprises a weekly clean in place(CIP) with 500 mg/L NaClO and 2000 mg/L citric acid, and a cleaningout of place (COP) with 1000 mg/L NaClO and 2000 mg/L citric acid,conducted twice yearly [30]. The above mentioned cleaning agents areusually corrosive or caustic, andmay damagemembranes. In this respect,mild and environmentally friendly cleaning agents, such as purifiedenzymes and surfactants, have been employed to extract biologically de-rived foulants from polymer membranes. Allie et al. [194] demonstratedthe feasibility of using both proteases and lipases to clean their UFmem-branes fouled by abattoir effluent. te Poele and van der Graaf [195]obtained 100% flux recovery for UF membranes by using new enzymaticcleaning protocol. Application of these agents in AnMBRs should befurther investigated.

5. Conclusions and perspectives

A critical analysis of literature reveals that much progress has beenachieved in applications and research of AnMBR technology. AnMBRtechnology features many advantages over aerobic treatment andconventional anaerobic methods, and the developments in membranematerials andmodules added to its advantages. The reviewalso demon-strates some advances in commercial AnMBR systems. AnMBRs appearto be suitable to treatmost of the streams, and high treatment efficiencyand high quality effluent was generally achieved, suggesting thatAnMBR technology was a prospective for wastewaters treatmentand subsequent reuse. Membrane fouling remained the major obstaclelimiting the widespread application of AnMBR. The literature results inmembrane fouling classification,mechanisms, affecting parameters andcontrol strategies were thereby summarized and updated.

All in all, the current review demonstrates the strong possibility andneed to enhance the use of AnMBR treating various streams. Despite therapid development of AnMBRs in recent years, there are remaining sev-eral barriers or challenges that limit their widespread practical applica-tion. Thus, further breakthroughs in these challenges should be pursuedin future works as summarized below:

• The literature reviewed revealed that most of the research reported onAnMBR treating wastewater is confined to bench-scale experiments.

Many times, results from bench testing could not simply transfer tofull-scale practical application. Further research is needed to supportits wide implementation at industrial scales.

• Membrane fouling and its consequences in terms of operating costsand plant maintenance remain the critical limiting factors affectingthe widespread application of AnMBRs for wastewater treatment.Although intensive efforts have been dedicated to the study onmem-brane fouling mechanisms and control, it is still necessary to developmore effective and easier methods to control and minimize mem-brane fouling especially in full-scale applications.

• The majority of membranes used in AnMBRs are UF and MF mem-branes, which represent significant costs of thewhole AnMBR system.Thus, adopting low costs filters in AnMBR for separation should be agood solution to reduce the costs of AnMBR. Efforts aimed in betterunderstanding on the filter properties as well as the influencing fac-tors would enable the optimization of their performance in AnMBRs.

• AnMBRs based on pressure-driven membrane processes for treatmentof wastewaters, especially municipal wastewater encountered twomajor challenges: membrane fouling, and low N and P removals. Therecent progress in FOmembrane process has provide a promising per-spective to resolve the above challenges since FO process has a lowerfouling propensity and can almost totally reject N and P contaminants.Continued efforts should be devoted to develop FOAnMBR system, andinvestigate its fouling behaviors and application in wastewater treat-ment. In addition, high costs of FO membrane should be reduced.

• Biogas recovery represents one of the major advantages of AnMBR.More engineering research needs to be directed toward biogas (mainlymethane) recovery measures. Development of effective and economi-cal methane recovery process would further improve economic feasi-bility of AnMBR for real wastewater treatment.

• It is operationally and economically advantageous to adopt anaerobic–aerobic processes in wastewater treatment. Such a process wouldcombine the benefits of membrane separation, anaerobic digestion(i.e. biogas production) and aerobic degradation (i.e. better COD andVSS removal). Attention should be paid on the research and applica-tion of the combined process.

• There is a short of fundamental information on the operational issues,cost issues, energy issues, and manufacture cost of AnMBR systemsfor various wastewaters treatment. Well-controlled pilot or full scaleAnMBR studies are needed to address these issues.

Above perspectives were proposed to the potential development oftheAnMBR technology in the future.Withmore efforts being conductedin both pilot- and full-scale AnMBR systems, the prospects of develop-ing technologically acceptable and economically feasible AnMBR treat-ment alternatives over conventional methods are pleasant.

Acknowledgments

Financial support of National Natural Science Foundation of China(nos. 51108424, 21275131, 21107099), Qianjiang Talent Project of


Zhejiang Province (no. 2012R10061), Zhejiang Provincial Departmentof Education Project (no. Y201121142) and Zhejiang Provincial NaturalScience Foundation (no. Y5110157) is highly appreciated.

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a review on anaerobic membrane bioreactors: applications

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